Sexology
by ohmytheon
Summary: (noun) "the study of human sexual life or relationships" - Stein is a scientist, after all, and so he likes to take a hands on approach to studying things. Luckily, he's got a more than consenting partner to help him learn.


**Author's Notes:** I'll have you know that I spent the better part of this blushing and trying not to giggle in a Starbucks. Why did I think it was a good idea to write smut in public? Why do I always do this? Also, it wasn't supposed to be this long, but Stein's brain runs about the same "never gonna stop" speed as mine.

 **Disclaimer:** I don't own anything.

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 **Sexology**

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Stein woke to the trickling of water in the shower and the muffled humming of the woman taking advantage of the small amount of warm water available in the Patchwork Lab. He lied still in the bed for a moment, completely stretched out, his feet at the very edge of the mattress. The ceiling fan spun lazily above him; any faster and it would wobble threateningly. As usual, he woke up completely devoid of covers, which he'd most likely thrown off of himself in the middle of the night. He wasn't used to sleeping so warmly.

Honestly, he still wasn't used to sleeping in a bed, much less under covers and _not alone_.

Lifting a hand up, Stein cranked the metal screw in his head, although he only did it once. Only months before, he would've had to crank it a few rounds before his mind began to clear. He couldn't remember a time when he had to only do it once. Maybe it was sleeping in the bed. He would have to continue to sleep in it for a while longer to test out his hypothesis before returning to sleeping at his desk.

While he would never say out loud that waking up to his Weapon's light singing was comforting, it did provide a sense of...stability. She always sang in the showers, whether she was home alone or not, although she made to keep her volume down if he was sleep or working. She was considerate in a way that he did not quite understand. She could've bellowed at the top of her lungs in the same room as him while he was working and he probably wouldn't even notice.

A minute later, he pulled himself out of the bed, stretching long and tall. In their apartment back when they were teenagers at the DWMA, he could almost touch the ceiling when he did that. Once, when they had partaken in too much libation as an experiment, she'd insisted that he carry her around his shoulders so that she could brush her fingers against the flaking ceiling, only for her to gasp and flinch away at how dirty it was because of his smoking. She'd toppled down on top of him; it had taken a while for them to untangle themselves.

There was a different sort of tangling now. Stein would not lie and say that he had never had sex with someone before, but once more, it was different. He had trouble saying how - the fact that he couldn't pinpoint the exact difference all the time was quite frustrating - but they didn't fight to untangle themselves from one another now. In fact, it was typically the opposite. The urge to blend with her now in more ways than one was strong.

He liked the way her soul beamed in his hands. It was always so open and vulnerable. Her trust in him didn't hurt so much now that the Kishin's Madness was gone. She knew that he would not be him if he did not have a little of his own Madness and he was who she said that she loved. It was a part of him as much as his soul was, as much as her soul was these days.

Making his way towards the bathroom, he quietly stepped inside. Sneaking up on people was a special talent of his, something that tended to creep everyone else out, and she was always so easy to sneak up on. Whether it was because of her single eye, her ability to get lost everywhere, or the fact that she easily got absorbed in something, he didn't know, but it amused him nonetheless. He liked the way she would gasp, how her heart rate would jump and her eye would widen - but her soul never fluttered. She could spot his soul a mile away once she was looking in that direction.

Steam clouded the small room, fogging up the mirror completely. A small smile creeped onto his face when he caught sight of her silhouette behind the shower curtain. She was so tiny, but there was no hiding the curves of her body. The first night after they crashed together, she confessed that she was insecure about those curves - how more than one man had commented on them as a bad thing or had tried to convince her to get rid of them. It had made him want to punch something, but that might have been the Madness talking. He liked the way his hands could trail up and down those curves and the way she'd tremble under his fingertips.

He was lucky that Marie was so involved in singing, holding a shampoo bottle and everything, the echoes of her voice hiding the sound of him stripping out of his boxers and slipping into the shower behind her. She was so easy to tease. His smile took on a little more of a demented look, but he knew that it wouldn't scare her.

When he finally placed his hands on her hips, Marie cut off singing and let out a squeak, dropping the shampoo and making a little jump. She slipped in the water, caught off guard, and probably would've fallen if not for the careful grip he had on her hips. After she steadied herself, she turned around, sliding in his grip, so that she could look at him. She wasn't wearing her eyepatch. The stitchwork, so similar to his own, seemed to stare up at him as well. Water splashed against them both.

"Stein, you scared me!" Marie accused, though she didn't sound angry. Her use of his surname told him that she was teasing him, but her heart was still beating hard in her chest, enough so that he could vaguely see it. She rolled her eye when she noticed where he was staring. "Did I wake you?"

"No," Stein replied honestly. He didn't know what woke him. He didn't want to say it was the lack of warmth after she left the bed, but… Well, change in body temperatures could wake a person up.

His answer seemed to please her. She settled a hand on his chest, a silly smile on her face, and then stepped back, gently guiding him into the spray of the water with her other hand on his back. Water plastered his grey hair down on his head, dripping into his eyes, but he didn't blink and remained focus on the way Marie breathed and how it almost looked like her soul was trying to shutter closer to his. It always did that when they became tangled with one another, like it was trying to resonate with his soul without her being in her Weapon form.

For some reason, it had a very powerful effect over him. He had to wonder if she knew what she was doing - he wouldn't put it past her to be so sly - but he found that he didn't care if she did.

Stein bent down in an angle that would've made anyone else uncomfortable, used to it from the way that he bent over his keyboard for hours. He nuzzled into her neck, the smell of her citrus shampoo body soap strong, and licked at the water slipping down her skin. She startled a little, her fingers curling against his back, but then let out a content hum when he moved to kiss her neck instead. He nibbled at her pulse point on her slim throat, still amazed every time she exposed herself to him like that. One hand stayed on her hip, massaging it softly, as his other slid up and to the left so that it rested on the swell of her belly.

She froze when his hand stopped there and pulled away slightly. Stein pulled back as well, locking eyes with her. It wasn't fear in her eyes so much as nervousness. Just because she wasn't afraid of him did not mean that she didn't feel jittery around him once in awhile. Her soul never wavered, but her mind did sometimes. She wasn't afraid of him so much as she was afraid of disappointing him.

How the little soul peeking out at him from underneath that swell of her belly could disappoint him was beyond him. Stein would be the first to say that he hadn't counted on ever being a father - and no amount of textbooks and scientific journals could prepare him - but as long as she was his partner in this, he was confident in the outcome.

"I know," Marie sighed, "I'm getting so _big_ …"

"You're pregnant," Stein pointed out matter-of-factly. "You have a life growing inside of you."

He didn't care if she was getting big; he would've been more concerned if she wasn't. He thought back to all the men that commented on her weight before in the past and wondered if he could track them down and give them a few good soul blasts. It would be fun and stress-relieving.

The smile that appeared on Marie's face was so tremulous that he thought she might cry. In order to keep that from happening, Stein moved to kiss her neck again, one of his favorite parts of her. She giggled as his hair brushed against her jaw, but then sucked in a gasp of air when he moved his hand from her growing belly to her best, swiping a calloused thumb over her nipple.

" _Oh_ ," she managed, and he was sure she knew exactly how little he cared about her getting bigger.

These days, Marie was typically the one to instigate sex of any kind. It was probably the hormones. One second she could be reading a book, playing with the fringe of his hair while he watched a documentary, and the next she was trying to crawl on top of him and make little work of his pants. Honestly, he didn't mind. It did strike him as odd that it didn't bother him when he could've sworn having his concentration interrupted for any reason would have irritated him before. After all, he wasn't like Spirit, who was ready to jump into any pretty woman's bed the second she showed him the slightest interest.

And yet there were times, like when he thought about the ways their souls seemed to resonate with one another outside of the typical soul resonance, when he couldn't help but start things. Waking up to her humming a song, almost missing her warmth at his side, scaring her in the shower… Maybe it was a little twisted, but that stunned look in her bright eyes got him going. It made him wonder if he could do anything else to make her gasp in shock.

Moving downward, Stein kissed down the hollow of her throat, sliding his tongue over her collarbone, which caused her to shimmy a little. Meanwhile, he moved his hip from her hip so that he was palming her ass. How any man could've complained about that, what with the way one side seemed to fit in the palm of his large hand, was a maddening quandary. Perhaps they just didn't have large enough hands. Perhaps they merely couldn't handle all of her. Despite her diminutive stature, Marie was no small thing. There was so much of her that there were nights when he felt like he couldn't get enough.

He kind of felt like that now, but now that he was awake, he was beginning to feel as hot as the water and more than just a tad bit hungry. It was such a different hunger from the one he'd come to know whenever any sort of Madness came over him. This one was no less stronger, but warmer as well, causing his soul to swell in a way that he could not describe.

Among other things, of course, but he could describe that easily. Biology could be very simple in the end.

Squeezing her ass, which made her press up against him, he moved to kiss down between the valley of her breasts before sliding his tongue over her other nipple. She made a little noise from the back of her throat, her nails digging into his back now, all of which he took as a good sign. He kept a light touch on her other nipple, knowing that the light touch of two different sensations would drive her mad. Indeed, it wasn't long before she began to squirm, her feet splashing in the water puddled in the tub.

Again, men complained about these curves? They were near anatomically perfect, in his opinion, and that was saying something. He could spend hours examining every inch of her, be it with his eyes, fingers, or his tongue.

Right as a whimper slipped out from her lips, Stein moved his attention elsewhere, continuing south. Her soul seemed to quake with anticipation and her heartbeat jumped as he kneeled down in front of her, pressing kisses down her belly and underneath it where her hipbone seemed to sit in between the swollen belly and her curvy hips. He kissed down to the middle, listening to the way her breathing began to pick up - and then jumped so that he was kissing her thighs instead. She took a staggering step back, frustrated with his change in course, but he merely grinned against her skin and bit her inner thigh, which made her squeak again.

He liked teasing her - made him feel all warm inside, in her words.

While kissing and slicking his tongue down one thigh and the other, Stein shuffled so that Marie was forced to lean back against the wall. She threaded one hand through his wet hair, seemingly content with just touching him, until he got too close to her center and moved away again. That was when she gave a sharp, reprimanding tug on his hair, causing him to still and bring his eyes to look up at her, a bit like one might look up at a god.

There was a tiny frown on her face. "Franken."

"Hm?"

Her one golden eye glowed with warning, letting him know that retribution would be sharp if he did not stop teasing her soon. He kind of liked the idea of her punishing him somehow. Ah, he'd always been a masochist and it was intriguing whenever their roles as Meister and Weapon switched.

But right now though, Stein really was more concerned with the task at hand. Let it be known that very little was capable of distracting him once he had his mind set on it - and right now, Marie was definitely on his mind.

This time when he kissed up her inner thigh, he did not pull away. When he slid a tongue up her center, she took in a gasp of air and slapped her free hand against the wall to steady herself. She had such a unique taste, the best taste, that he tried to describe on more than one occasion. Apparently, trying to come up with proper description of her exact flavor wasn't one of the most romantic things he'd done, especially since he'd accidentally left his notes on the coffee table where Spirit stumbled across it and thought it was about some sort of weird pie.

He sucked on her gently at first, doing his best not to get out of hand. It was sometimes difficult not to lose himself when he went down on her. He focused on her ragged breathing, the way she pulled on his hair and then let go of him, like she couldn't exactly control her movements. Her hips moved in tandem with his mouth, but he could tell that she was holding herself back so that she wouldn't buck into his face.

When he pushed his tongue inside of her, a loud moan tumbled out of her mouth. Surprisingly, unlike in the rest of her life, she tried to be quiet during sex a lot of the time, perhaps out of embarrassment. Stein didn't want that though. He liked to do everything in his power to drag those noises out of her, to make her buck against him, drag her nails down his back. He didn't want her to hold back for anything, anyone, or any reason. It'd be amusing if she was loud enough for Spirit to hear all the way back at his place to give him a taste of his own medicine.

That led to an interesting thought. Maybe Stein should instigate sex with Marie the next time they were over at Spirit's apartment. It would serve him right for stumbling to the Lab with a girl from ChupaCabra at three in the morning a few weeks ago because he'd lost his keys. The Death Scythe had interrupted them right when Marie was feeling particularly feisty.

Hearing her whimper and feeling her move her hips more, Stein put his focus back on her clit, which was already sensitive enough to cause her to practically melt as he sucked on it. She was wet enough so that when he pumped a finger inside of her it glided in effortlessly. He added a second one when she groaned a demand of, "More," always one to oblige his Weapon. She had more experience with herself than him, after all, though he was catching up.

He pulled his hand away from her ass so that he could grab her from behind her right knee and then pulled on it so that he could hook it over his shoulder. This gave him much easier access to her center and he was able to suck and lick on her much more vigorously, pumping his fingers inside of her in tandem. The sudden change in position and dialed up aggressiveness on his part caused her to jerk and cry out. He could feel the walls of her vagina clenching around him, and he felt a strange sense of jealousy towards his own fingers. Honestly he'd prefer to bury himself in her right now, but he wasn't about to stop either.

"Franken, I-"

"Mmhm?" he hummed against her, the vibrations against her clit causing her to gasp.

She tugged on his hair again, this time letting him know not to stop. "I think-" She couldn't stop herself from bucking her hips against his face if she tried. More whimpers escaped from her in between gasps as she tried to breathe and tension built up inside of her body, almost too much for her to take. "Franken- oh, OH-"

A string of words that didn't make any sense and moans tumbled out as she came, her orgasm running through her body. He didn't stop and instead picked up his pace. She cried out, pulling his hair painfully, but he didn't care. How could he possibly describe what she tasted and smelled like in this moment? He was a little grateful that he hadn't been inside of her right now, if only because of how strong her orgasm felt. He probably would've come right then had been. When Marie orgasmed, it was like a tidal wave. Her soul glowed so powerfully, itching towards his again, wanting to be one.

He knew the feeling well.

After a while, Marie came to, her movements slowing and her breathing steadying. Her heart pounded to the point where it made it look like her chest was trembling. He pulled his fingers out, giving her one more languid lick, before pulling away and popping his neck. Her hand fell away from his head as she reached for one of his hands, sliding her fingers in between his, and he stood up so that she could nuzzle against him. The water was more lukewarm than anything after all of that, but it didn't bother him. He let her lay against him as press tiny kisses against his chest.

"Marie?"

"Yes?" she mumbled.

When he didn't say anything right away, she gazed up at him. The look in his eyes must have been something because she actually blushed, despite the fact that he'd just been sucking on her. She barely had the time to turn the water off before he scooped her up and stepped out of the shower, carrying her out of the bathroom bridal shower.

"Franken, we're wet!" Marie squealed before he deposited her on the bed and crawled over top of her, pressing her down into the mattress with his weight and sliding his hands over her again.

He didn't rightly give a damn about whether they hadn't bothered to dry off from the shower. The bed would dry, but he wasn't about to let her do the same. He didn't have the patience to wait any longer. And being as skilled with his Weapon as he was, it wasn't long before he had her crying out again, moaning his name, and clinging to him. He was never loud and kept quiet most of the time, which didn't bother her, but even he couldn't stop himself from groaning as he buried himself in her.

Hormones. It had to be the hormones. Maybe her being pregnant did something to him as well. Something about pheromones in the air. He'd have to look it up later.

All he knew right now was that there was something demanding and consuming about tangling with Marie like this, most notably in the morning when he just woke up. It would take some more investigating and studying for him to find out why. He didn't mind though; he liked experiments, especially hands on ones.


End file.
